Conventionally, in Japan, there is a system for providing traffic information about road traffic situations to vehicles, which is called VICS (Vehicle Information and Communication System) run by Vehicle Information and Communication System Center (hereafter, referred to as VICS (registered trademark) center). The VICS gathers information in real time sent from road sensors (ultrasonic sensors, loop coil sensors, AVI, optical beacon units, etc.) managed by prefectural polices and road managers, collects the traffic information, such as traffic jam and travel time information, in one centralized location, and provides the information to drivers. The traffic information update period normally takes five minutes since it depends on the collection period of measured information received from road sensors, and it is possible to provide information all the time as long as road sensors operate properly. As information providing means, narrow-area communication that uses beacon units (optical, radio wave) and wide-area communication by FM multiplex broadcasting are popular. Drivers who use car navigation apparatus can acquire real-time traffic information provided by the VICS, select a route that avoids a traffic jam, and estimate arrival time required to the destination.
Since the road link of which traffic information can be provided by the VICS depends on the presence or absence of road sensors, traffic information that can be provided is limited to predetermined links. Hereafter, this link is referred to as a VICS link. The number of VICS links is increasing year by year due to new installation of road sensors, and development of probe technology for traffic situations enables traffic information about links other than VICS links to be collected and provided. That is, the amount of traffic information delivered from the traffic information providing center to car navigation apparatuses is expected to continuously increase. Accordingly, from now on, it is required to reduce the amount of information delivered from the traffic information providing center to in-vehicle information apparatuses.
Incidentally, in Japanese Patent Laid-open No. 2006-84257, a technology that uses difference delivery has been disclosed as a method for reducing the amount of traffic information delivered from the traffic information delivery center to in-vehicle information apparatuses typified by car navigation apparatuses. In the difference delivery procedure, when traffic information is delivered, only traffic information (difference information) which is different from the previously delivered traffic information is delivered, thereby making it possible to reduce the amount of information to be delivered.